Current state of the art for storage and transfer of natural gas (NG) to engines used in mobile off road applications consist primarily of commonly available components and existing technology configured for demonstration. The storage tanks and support equipment used in existing systems are repurposed from other industries in commercially available sizes and configurations that are less than ideal for high horsepower, mobile applications. For locomotive applications, these systems are made up of dedicated NG tender cars, or standard tanks mounted on flat railcars or fixed underframe between the trucks.
As a result, existing systems for locomotives, both CNG and LNG have been limited to specific local application and pilot or research projects that do not necessarily require large storage capacity, ease of service, maintainability or adherence to regulations such as crash worthiness necessary for general use on freight and passenger lines. In off road mobile applications, the most significant technical challenge is attaining the necessary DGE capacity to motivate manufacturers and end users to convert from diesel fuel. In packaging of the NG tanks to attain a sufficiently high storage capacity and required crashworthiness, it is still important that the system still be simple, reliable and easy to maintain. Further this should be done with minimal or no perceived impact to the end user.
For tenders and flat cars, it is possible to attain acceptable storage capacity, but results in additional rail cars to the train consist and/or added weight and complexity for the operators to deal with. Tanker cars or tanks on flat beds must be modified by adding structure and reinforcement necessary in order to make them meet safety and crashworthiness requirements for tender car use. For underframe storage, these existing components and packaging schemes limit the storage density, thus limiting the range, performance and functionality of the vehicle the system is supporting. Underframe or belly mounted tanks in place of or in addition to the existing fuel tanks on the locomotive present greater challenges in package and storage capacity as well as introducing additional requirements for crash worthiness criteria and other safety regulations. Current underframe NG storage systems have been limited to 200 DGE (diesel gallon equivalent). In order for natural gas fueled solutions to be commercially viable for locomotives, 600 DGE or more must be attained. Prior locomotive tank systems such as the CNG system on the Napa Valley Wine train have conventional type 1 steel tanks installed under frame longitudinally on the locomotive along the axis of train motion. This approach makes mounting and servicing the tanks less than ideal as it requires removing the enclosure with all the tanks at once as one assembly. Another downside of this type of longitudinal tank arrangement is its lack of storage density when the longitudinal space to put the tanks in is less than 9 feet as it is with typical switcher locomotives. This results in a higher quantity of shorter tanks, reducing the storage capacity and increasing the cost as well as the number of components and potential leak points.
For both tender car and underframe NG storage, maximizing capacity requires close spacing of the NG cylinders making mounting the tanks in tender cars or under frame enclosures a challenge. There are currently two common methods of mounting NG cylinders, strap mounting with two steel straps wrapped around the outer diameter of the tank, or neck mounting with block mounts capturing a large pin at each end of the tank. In strap mounting, rubber isolator strips are placed between the mount straps and the surface of the tank to both prevent damage to the outer composite skin, and to allow the tanks to grow slightly in length and diameter when they are filled. In pin mounting, one end of the tank is fixed into position to prevent rotation, while the other end has a loose fitting mount block which allows the pin to slide axially as the tank grows in length during filling.
The most common application of CNG cylinders such as these are municipal transit buses. In transit buses the tanks are slightly shorter than proposed laterally mounted locomotive tanks and they are typically neck mounted. Neck mounting would appear to offer the easiest mounting in the tight locomotive enclosure, but there are distinct and important differences between the switcher duty cycle and the transit bus cycle. The switcher has metal on metal wheels and will see significantly more vertical vibration, this will affect the loose fit in the sliding pin block causing degradation of the pin material where it meets the block, as the locomotive CNG tank is longer and heavier it could also cause fatigue in the aluminum neck boss material and a tank failure at the tank neck. Another important factor is the life cycle; buses are in service typically 10 to 12 years, whereas 50 year old switchers are still being rebuilt today. The most significant detriment to the neck mounting system is the operation of the switcher locomotive, its primary purpose is to frequently couple and uncouple with rail cars, and these coupling events lead to repeated impulse loads of several g's. These repeated impulses could be a fatigue risk to conventional neck mounted composite CNG cylinders if the loads are significant and perpendicular to the axis of the tank.
Strap mounting of the CNG cylinders would overcome this impulse loading issue. The issue with strap mounting of CNG cylinders is that access is needed to tighten the straps from the side and this will require spacing the tanks further apart thus reducing the total number of CNG cylinders and total fuel capacity.
As natural gas has been introduced as a fuel for the railroad industry there has been a focus on using Liquefied natural gas (LNG) as the storage medium for increased range over using compressed natural gas (CNG). LNG was also considered the best fuel storage system for transit buses when natural gas was first used for that application. Most of the original LNG transit buses have since been replaced by CNG as the technology of the CNG tanks has improved. LNG currently has many disadvantages over CNG. LNG plants have to be built very large to economically make LNG, up until 2012 there were only 3 LNG plants constructed specifically to make LNG for transportation use. LNG tanks absorb heat from the outside and if the tanks are left to sit they will vent methane gas. This becomes a problem when an agency has several hundred busses with some of them idle. Also as LNG is stored for a long period of time and is venting, its percentage of methane will drop over time as methane boils off first. This effect is called weathering and as the fuel weathers its octane rating decreases as the percentage of methane decreases and its percentage of lower octane propane, butane and other ‘thanes’ increases. Using fuel that has weathered too long can result in engine damage.
Current mobile LNG storage and delivery systems for natural gas fueled engines are passive and controlled by regulating the vaporization of the LNG to CNG at the necessary pressure for the fuel system application it is supporting. Using this approach, the LNG in the tank is maintained at or near the vaporization temperature for the natural gas at the required supply pressure either by removing LNG from the tank to cool and lower the pressure or heating the tank externally to allow more vaporization to raise the pressure. The vaporization based systems are typically designed to operate at approximately 125 psi and must maintain a specific temperature range of −130 +/−10 degrees Celsius in order to provide sufficient fuel pressure to the engines fuel management system. As a result, using the vaporization point of the natural gas to control fuel delivery is a proven concept but it reduces the amount of LNG that can be stored in a given tank, the pressure it is supplied at, and the time that it can be maintained in liquid state within the fixed volume of the tank before it vaporizes and must be purged. Further this system requires heat to be added to the tank to raise the pressure or off gassing or flaring of vaporized natural gas if it is not consumed in time by the engine, both of which expend energy or release fuel which further reduced range and efficiency
In order to refuel a system like this, the filling equipment typically saturates the fuel by adding heat. If a vehicle is not filled with saturated fuel, it may take a significant amount of time to raise the equalized saturation temperature and pressure in the tank so that the vehicle engine can be operated at full power. This is especially a problem when refueling large pieces of offload equipment from LNG tanker trucks.
A further challenge in passive LNG tank systems is maintaining the required tank pressure when the engine is at high loads and consuming a large amount of fuel over an extended time. When LNG fuel is withdrawn from the tank, a small amount of the remaining LNG will boil off to bring the tank pressure back to equilibrium. When this small amount of fuel boils off, it absorbs a large amount of latent heat and chills the remaining liquid. This was a particular problem with the LNG switchers at LA Junction RR, the locomotives could only operate at high power for a short time before low LNG tank pressure forced the system to reduce engine power output.